This invention relates to a method for forming and shaping stiff plasticized powder mixtures containing a binder, solvent for the binder, surfactant, and a component in which at least the binder, its solvent, and the powder materials are insoluble, referred to as the non-solvent. The solvent is present in an amount that is less than would be present without the non-solver. The combinations of mixture components result in improved wet green strength in the green body than otherwise occurs. Furthermore, this increase in wet green strength occurs without proportional increases in pressure or torque. Also, in extrusion of the above mixtures, the shape of the extrudate or green body is retained at the very instant it exits the die, with no time delay.
Powder mixtures having a cellulose ether binder are used in forming articles of various shapes. For example ceramic powder mixtures are formed into honeycombs which are used as substrates in catalytic and adsorption applications. The mixtures must be well blended and homogeneous in order for the resulting body to have good integrity in size and shape and uniform physical properties. The mixtures have organic additives in addition to the binders. These additives can be surfactants, lubricants, and dispersants and function as processing aids to enhance wetting thereby producing a uniform batch.
A major and ongoing need in extrusion of bodies from highly filled powder mixtures, especially multicellular bodies such as honeycombs is to extrude a stiffer body without causing proportional increase in pressures. The need is becoming increasingly critical as thinner walled higher cell density cellular structures are becoming more in demand for various applications. Thin walled products with current technology are extremely difficult to handle without causing shape distortion.
Rapid-setting characteristics are important for honeycomb substrates. If the cell walls of the honeycomb can be solidified quickly after forming, the dimension of the greenware will not be altered in subsequent cutting and handling steps. This is especially true for a fragile thin-walled or complex shaped product, or a product having a large frontal area.
Prior rapid stiffening methods involve time-delayed stiffening using rapid set waxes as disclosed, for example in U.S. Pat. No. 5,568,652, and/or applying an external field such as an electrical, ultrasonic, or RF field at the die exit. All of these methods involve extrusion of soft batches. Historically, for highly filled ceramic mixtures, soft batches have lead to better extrusion quality. Attempts to extrude stiffer ceramic batches with the current batch components, i.e. cellulose ether binder, lowering the amount of water and/or additives such as sodium tallowate or sodium stearate have not been very successful because of the higher extrusion pressures resulting from collision of finer particles, and the abrasiveness of the materials involved.
The growing need for thinner webs (1-2 mil)/high density cellular products to be extruded to shape necessitates stiffening at the very instant the batch exits the die.
The present invention fills the need for instantaneous forming of stiff batches which is especially beneficial for extrusion of thin walled honeycombs, and shape retention of extruded bodies at the very instant the batch exits the die.
In accordance with one aspect of the invention, there is provided powder mixtures and a method of forming and shaping the mixtures. The method involves compounding the components of powder materials, binder, solvent for the binder, surfactant, and non-solvent with respect to at least the binder, the solvent, and the powder materials. The non-solvent is lower in viscosity than the binder combined with the solvent. The solvent is present in an amount that is less than the amount that would be present otherwise. The components are mixed and plasticized, and shaped to form a green body. The choice of components results in improved wet green strength in the green body.
The method is especially useful for extrusion processing of aqueous binder systems such as water and cellulose ethers and hydrophobic non-solvents, to form structures such as honeycombs.
In accordance with another aspect of the invention, there is provided a honeycomb body having a ratio of the isostatic strength to A-axis strength of at least about 0.33, upon firing.